A User Equipment (UE), also known as a mobile station, wireless terminal and/or mobile terminal is enabled to communicate wirelessly in a wireless communication network, sometimes also referred to as a cellular radio system. The communication may be made, e.g., between user equipments, between a user equipment and a wire connected telephone and/or between a user equipment and a server via a Radio Access Network (RAN) and possibly one or more core networks. The wireless communication may comprise various communication services such as voice, messaging, packet data, video, broadcast, etc
The user equipment may further be referred to as mobile telephone, cellular telephone, computer tablet or laptop with wireless capability, etc. The user equipment in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another user equipment, a stationary entity or a server.
The wireless communication network covers a geographical area which is divided into cell areas, with each cell area being served by a radio network node or base station, e.g., a Radio Base Station (RBS) or Base Transceiver Station (BTS), which in some networks may be referred to as “eNB”, “eNodeB”, “NodeB” or “B node”, depending on the technology and/or terminology used.
Sometimes, the expression “cell” may be used for denoting the radio network node itself. However, the cell may also in normal terminology be used for the geographical area where radio coverage is provided by the radio network node at a base station site. One radio network node, situated on the base station site, may serve one or several cells. The radio network nodes may communicate over the air interface operating on radio frequencies with any user equipment within range of the respective radio network node.
In some radio access networks, several radio network nodes may be connected, e.g., by landlines or microwave, to a Radio Network Controller (RNC), e.g., in Universal Mobile Telecommunications System (UMTS). The RNC, also sometimes termed Base Station Controller (BSC), e.g., in GSM, may supervise and coordinate various activities of the plural radio network nodes connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), radio network nodes, which may be referred to as eNodeBs or eNBs, may be connected to a gateway, e.g., a radio access gateway, to one or more core networks. LTE is based on the GSM/EDGE and UMTS/HSPA network technologies, increasing the capacity and speed using a different radio interface together with core network improvements.
LTE-Advanced, i.e. LTE Release10 and later releases are set to provide higher bitrates in a cost efficient way and, at the same time, completely fulfil the requirements set by International Telecommunication Union (ITU) for the International Mobile Telecommunications (IMT)-Advanced, sometimes also referred to as 4G (abbreviation for “fourth generation”). Beyond 3G mobile communication systems, such as e.g., 3GPP LTE, offer high data rate in the downlink by employing multiple antenna systems utilising Multiple-Input and Multiple-Output (MIMO).
Massive MIMO is a new technology that uses large Antenna Arrays Systems (AAS) with individual transceivers to dramatically improve throughput of wireless communication systems. The benefit of these large arrays is the ability to spatially resolve and separate received and transmitted signals with very high resolution. Typical limitation factors like Noise and Interference is reduced to the extent that previously negligible effects become limiting. One of the most difficult of these effects is Pilot Contamination. Pilot Contamination is caused by lost or lack of training sequence orthogonality between cells.
Massive MIMO is sometimes loosely defined as a system comprising e.g. 100 or more transceivers. Various investigations in this community have shown Massive MIMO systems that benefit from several hundred's of transceivers.
A Massive MIMO enabled radio network node estimates the radio channel from user equipments by correlating the received signal with a known signal transmitted by the user equipments, i.e. pilot signals. These pilot signals are made orthogonal to each other. This means that the result of correlation performed by the radio network node during training will only contain a systematic response from desired link form the mobile transmitting the pilot used in the correlation.
In the present context, the expressions downlink, downstream link or forward link may be used for the transmission path from the radio network node to the user equipment. The expression uplink, upstream link or reverse link may be used for the transmission path in the opposite direction, i.e., from the user equipment to the radio network node.
However, there are only a limited set of orthogonal pilot signals available. This means that the same pilot signal has to be reused to provide enough training time and accurate channel estimations. This will cause increased pilot contamination within the cell, which will deteriorate the signal quality and thereby also the signalling throughput within the wireless communication system.
Therefore, it is desired to reduce the problems with pilot contamination in Massive MIMO systems, in order to make use of such systems feasible.